Github Pages prerelease folders

**Is your feature request related to a problem? Please describe.**

Not related to a problem :)

**Describe the solution you'd like**

Some repositories managed by `auto` are only designed for creating a Github Pages page. When I create a prerelease or canary branch, I would want to be able to allow other people to see the release for testing or bug reporting.

I propose that prerelease branches should push to a subfolder that matches the prerelease branch name in the specified Github Pages URL (ex. pushes to the `next` branch will deploy to `pages.url/next`.

**Describe alternatives you've considered**

I have considered attempting to have a separate repo to send a canary release to, but don't see that as an effective long-term solution.